src/smpi/colls/reduce/reduce-ompi.cpp

   1 /* Copyright (c) 2013-2019. The SimGrid Team.
   2  * All rights reserved.                                                     */
   3
   4 /* This program is free software; you can redistribute it and/or modify it
   5  * under the terms of the license (GNU LGPL) which comes with this package. */
   6
   7 /*
   8  * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
   9  *                         University Research and Technology
  10  *                         Corporation.  All rights reserved.
  11  * Copyright (c) 2004-2009 The University of Tennessee and The University
  12  *                         of Tennessee Research Foundation.  All rights
  13  *                         reserved.
  14  * Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
  15  *                         University of Stuttgart.  All rights reserved.
  16  * Copyright (c) 2004-2005 The Regents of the University of California.
  17  *                         All rights reserved.
  18  *
  19  * Additional copyrights may follow
  20  */
  21
  22 #include "../coll_tuned_topo.hpp"
  23 #include "../colls_private.hpp"
  24
  25 namespace simgrid{
  26 namespace smpi{
  27
  28 int smpi_coll_tuned_ompi_reduce_generic(const void* sendbuf, void* recvbuf, int original_count,
  29                                     MPI_Datatype datatype, MPI_Op  op,
  30                                     int root, MPI_Comm comm,
  31                                     ompi_coll_tree_t* tree, int count_by_segment,
  32                                     int max_outstanding_reqs );
  33 /**
  34  * This is a generic implementation of the reduce protocol. It used the tree
  35  * provided as an argument and execute all operations using a segment of
  36  * count times a datatype.
  37  * For the last communication it will update the count in order to limit
  38  * the number of datatype to the original count (original_count)
  39  *
  40  * Note that for non-commutative operations we cannot save memory copy
  41  * for the first block: thus we must copy sendbuf to accumbuf on intermediate
  42  * to keep the optimized loop happy.
  43  */
  44 int smpi_coll_tuned_ompi_reduce_generic(const void* sendbuf, void* recvbuf, int original_count,
  45                                     MPI_Datatype datatype, MPI_Op  op,
  46                                     int root, MPI_Comm comm,
  47                                     ompi_coll_tree_t* tree, int count_by_segment,
  48                                     int max_outstanding_reqs )
  49 {
  50     char *inbuf[2] = {NULL, NULL}, *inbuf_free[2] = {NULL, NULL};
  51     char *accumbuf = NULL, *accumbuf_free = NULL;
  52     char *local_op_buffer = NULL, *sendtmpbuf = NULL;
  53     ptrdiff_t extent, lower_bound, segment_increment;
  54     MPI_Request  reqs[2] = {MPI_REQUEST_NULL, MPI_REQUEST_NULL};
  55     int num_segments, line, ret, segindex, i, rank;
  56     int recvcount, prevcount, inbi;
  57
  58     /**
  59      * Determine number of segments and number of elements
  60      * sent per operation
  61      */
  62     datatype->extent(&lower_bound, &extent);
  63     num_segments = (original_count + count_by_segment - 1) / count_by_segment;
  64     segment_increment = count_by_segment * extent;
  65
  66     sendtmpbuf = (char*) sendbuf;
  67     if( sendbuf == MPI_IN_PLACE ) {
  68         sendtmpbuf = (char *)recvbuf;
  69     }
  70
  71     XBT_DEBUG("coll:tuned:reduce_generic count %d, msg size %lu, segsize %lu, max_requests %d", original_count,
  72               (unsigned long)(num_segments * segment_increment), (unsigned long)segment_increment,
  73               max_outstanding_reqs);
  74
  75     rank = comm->rank();
  76
  77     /* non-leaf nodes - wait for children to send me data & forward up
  78        (if needed) */
  79     if( tree->tree_nextsize > 0 ) {
  80         ptrdiff_t true_extent, real_segment_size;
  81         true_extent=datatype->get_extent();
  82
  83         /* handle non existant recv buffer (i.e. its NULL) and
  84            protect the recv buffer on non-root nodes */
  85         accumbuf = (char*)recvbuf;
  86         if( (NULL == accumbuf) || (root != rank) ) {
  87             /* Allocate temporary accumulator buffer. */
  88             accumbuf_free = (char*)smpi_get_tmp_sendbuffer(true_extent +
  89                                           (original_count - 1) * extent);
  90             if (accumbuf_free == NULL) {
  91                 line = __LINE__; ret = -1; goto error_hndl;
  92             }
  93             accumbuf = accumbuf_free - lower_bound;
  94         }
  95
  96         /* If this is a non-commutative operation we must copy
  97            sendbuf to the accumbuf, in order to simplfy the loops */
  98         if ((op != MPI_OP_NULL && not op->is_commutative())) {
  99           Datatype::copy((char*)sendtmpbuf, original_count, datatype, (char*)accumbuf, original_count, datatype);
 100         }
 101         /* Allocate two buffers for incoming segments */
 102         real_segment_size = true_extent + (count_by_segment - 1) * extent;
 103         inbuf_free[0] = (char*) smpi_get_tmp_recvbuffer(real_segment_size);
 104         if( inbuf_free[0] == NULL ) {
 105             line = __LINE__; ret = -1; goto error_hndl;
 106         }
 107         inbuf[0] = inbuf_free[0] - lower_bound;
 108         /* if there is chance to overlap communication -
 109            allocate second buffer */
 110         if( (num_segments > 1) || (tree->tree_nextsize > 1) ) {
 111             inbuf_free[1] = (char*) smpi_get_tmp_recvbuffer(real_segment_size);
 112             if( inbuf_free[1] == NULL ) {
 113                 line = __LINE__; ret = -1; goto error_hndl;
 114             }
 115             inbuf[1] = inbuf_free[1] - lower_bound;
 116         }
 117
 118         /* reset input buffer index and receive count */
 119         inbi = 0;
 120         recvcount = 0;
 121         /* for each segment */
 122         for( segindex = 0; segindex <= num_segments; segindex++ ) {
 123             prevcount = recvcount;
 124             /* recvcount - number of elements in current segment */
 125             recvcount = count_by_segment;
 126             if( segindex == (num_segments-1) )
 127                 recvcount = original_count - count_by_segment * segindex;
 128
 129             /* for each child */
 130             for( i = 0; i < tree->tree_nextsize; i++ ) {
 131                 /**
 132                  * We try to overlap communication:
 133                  * either with next segment or with the next child
 134                  */
 135                 /* post irecv for current segindex on current child */
 136                 if( segindex < num_segments ) {
 137                     void* local_recvbuf = inbuf[inbi];
 138                     if( 0 == i ) {
 139                         /* for the first step (1st child per segment) and
 140                          * commutative operations we might be able to irecv
 141                          * directly into the accumulate buffer so that we can
 142                          * reduce(op) this with our sendbuf in one step as
 143                          * ompi_op_reduce only has two buffer pointers,
 144                          * this avoids an extra memory copy.
 145                          *
 146                          * BUT if the operation is non-commutative or
 147                          * we are root and are USING MPI_IN_PLACE this is wrong!
 148                          */
 149                         if(  (op==MPI_OP_NULL || op->is_commutative()) &&
 150                             !((MPI_IN_PLACE == sendbuf) && (rank == tree->tree_root)) ) {
 151                             local_recvbuf = accumbuf + segindex * segment_increment;
 152                         }
 153                     }
 154
 155                     reqs[inbi]=Request::irecv(local_recvbuf, recvcount, datatype,
 156                                              tree->tree_next[i],
 157                                              COLL_TAG_REDUCE, comm
 158                                              );
 159                 }
 160                 /* wait for previous req to complete, if any.
 161                    if there are no requests reqs[inbi ^1] will be
 162                    MPI_REQUEST_NULL. */
 163                 /* wait on data from last child for previous segment */
 164                 Request::waitall( 1, &reqs[inbi ^ 1],
 165                                              MPI_STATUSES_IGNORE );
 166                 local_op_buffer = inbuf[inbi ^ 1];
 167                 if( i > 0 ) {
 168                     /* our first operation is to combine our own [sendbuf] data
 169                      * with the data we recvd from down stream (but only
 170                      * the operation is commutative and if we are not root and
 171                      * not using MPI_IN_PLACE)
 172                      */
 173                     if( 1 == i ) {
 174                         if( (op==MPI_OP_NULL || op->is_commutative())&&
 175                             !((MPI_IN_PLACE == sendbuf) && (rank == tree->tree_root)) ) {
 176                             local_op_buffer = sendtmpbuf + segindex * segment_increment;
 177                         }
 178                     }
 179                     /* apply operation */
 180                     if(op!=MPI_OP_NULL) op->apply( local_op_buffer,
 181                                    accumbuf + segindex * segment_increment,
 182                                    &recvcount, datatype );
 183                 } else if ( segindex > 0 ) {
 184                     void* accumulator = accumbuf + (segindex-1) * segment_increment;
 185                     if( tree->tree_nextsize <= 1 ) {
 186                         if(  (op==MPI_OP_NULL || op->is_commutative()) &&
 187                             !((MPI_IN_PLACE == sendbuf) && (rank == tree->tree_root)) ) {
 188                             local_op_buffer = sendtmpbuf + (segindex-1) * segment_increment;
 189                         }
 190                     }
 191                     if(op!=MPI_OP_NULL) op->apply( local_op_buffer, accumulator, &prevcount,
 192                                    datatype );
 193
 194                     /* all reduced on available data this step (i) complete,
 195                      * pass to the next process unless you are the root.
 196                      */
 197                     if (rank != tree->tree_root) {
 198                         /* send combined/accumulated data to parent */
 199                         Request::send( accumulator, prevcount,
 200                                                   datatype, tree->tree_prev,
 201                                                   COLL_TAG_REDUCE,
 202                                                   comm);
 203                     }
 204
 205                     /* we stop when segindex = number of segments
 206                        (i.e. we do num_segment+1 steps for pipelining */
 207                     if (segindex == num_segments) break;
 208                 }
 209
 210                 /* update input buffer index */
 211                 inbi = inbi ^ 1;
 212             } /* end of for each child */
 213         } /* end of for each segment */
 214
 215         /* clean up */
 216         smpi_free_tmp_buffer(inbuf_free[0]);
 217         smpi_free_tmp_buffer(inbuf_free[1]);
 218         smpi_free_tmp_buffer(accumbuf_free);
 219     }
 220
 221     /* leaf nodes
 222        Depending on the value of max_outstanding_reqs and
 223        the number of segments we have two options:
 224        - send all segments using blocking send to the parent, or
 225        - avoid overflooding the parent nodes by limiting the number of
 226        outstanding requests to max_oustanding_reqs.
 227        TODO/POSSIBLE IMPROVEMENT: If there is a way to determine the eager size
 228        for the current communication, synchronization should be used only
 229        when the message/segment size is smaller than the eager size.
 230     */
 231     else {
 232
 233         /* If the number of segments is less than a maximum number of oustanding
 234            requests or there is no limit on the maximum number of outstanding
 235            requests, we send data to the parent using blocking send */
 236         if ((0 == max_outstanding_reqs) ||
 237             (num_segments <= max_outstanding_reqs)) {
 238
 239             segindex = 0;
 240             while ( original_count > 0) {
 241                 if (original_count < count_by_segment) {
 242                     count_by_segment = original_count;
 243                 }
 244                 Request::send((char*)sendbuf +
 245                                          segindex * segment_increment,
 246                                          count_by_segment, datatype,
 247                                          tree->tree_prev,
 248                                          COLL_TAG_REDUCE,
 249                                          comm) ;
 250                 segindex++;
 251                 original_count -= count_by_segment;
 252             }
 253         }
 254
 255         /* Otherwise, introduce flow control:
 256            - post max_outstanding_reqs non-blocking synchronous send,
 257            - for remaining segments
 258            - wait for a ssend to complete, and post the next one.
 259            - wait for all outstanding sends to complete.
 260         */
 261         else {
 262
 263             int creq = 0;
 264             MPI_Request* sreq = new (std::nothrow) MPI_Request[max_outstanding_reqs];
 265             if (NULL == sreq) { line = __LINE__; ret = -1; goto error_hndl; }
 266
 267             /* post first group of requests */
 268             for (segindex = 0; segindex < max_outstanding_reqs; segindex++) {
 269                 sreq[segindex]=Request::isend((char*)sendbuf +
 270                                           segindex * segment_increment,
 271                                           count_by_segment, datatype,
 272                                           tree->tree_prev,
 273                                           COLL_TAG_REDUCE,
 274                                           comm);
 275                 original_count -= count_by_segment;
 276             }
 277
 278             creq = 0;
 279             while ( original_count > 0 ) {
 280                 /* wait on a posted request to complete */
 281                 Request::wait(&sreq[creq], MPI_STATUS_IGNORE);
 282                 sreq[creq] = MPI_REQUEST_NULL;
 283
 284                 if( original_count < count_by_segment ) {
 285                     count_by_segment = original_count;
 286                 }
 287                 sreq[creq]=Request::isend((char*)sendbuf +
 288                                           segindex * segment_increment,
 289                                           count_by_segment, datatype,
 290                                           tree->tree_prev,
 291                                           COLL_TAG_REDUCE,
 292                                           comm );
 293                 creq = (creq + 1) % max_outstanding_reqs;
 294                 segindex++;
 295                 original_count -= count_by_segment;
 296             }
 297
 298             /* Wait on the remaining request to complete */
 299             Request::waitall( max_outstanding_reqs, sreq,
 300                                          MPI_STATUSES_IGNORE );
 301
 302             /* free requests */
 303             delete[] sreq;
 304         }
 305     }
 306     ompi_coll_tuned_topo_destroy_tree(&tree);
 307     return MPI_SUCCESS;
 308
 309  error_hndl:  /* error handler */
 310     XBT_DEBUG("ERROR_HNDL: node %d file %s line %d error %d\n",
 311                    rank, __FILE__, line, ret );
 312     smpi_free_tmp_buffer(inbuf_free[0]);
 313     smpi_free_tmp_buffer(inbuf_free[1]);
 314     smpi_free_tmp_buffer(accumbuf);
 315     return ret;
 316 }
 317
 318 /* Attention: this version of the reduce operations does not
 319    work for:
 320    - non-commutative operations
 321    - segment sizes which are not multiplies of the extent of the datatype
 322      meaning that at least one datatype must fit in the segment !
 323 */
 324
 325
 326 int Coll_reduce_ompi_chain::reduce(const void *sendbuf, void *recvbuf, int count,
 327                                         MPI_Datatype datatype,
 328                                         MPI_Op  op, int root,
 329                                         MPI_Comm  comm
 330                                         )
 331 {
 332     uint32_t segsize=64*1024;
 333     int segcount = count;
 334     size_t typelng;
 335     int fanout = comm->size()/2;
 336
 337     XBT_DEBUG("coll:tuned:reduce_intra_chain rank %d fo %d ss %5u", comm->rank(), fanout, segsize);
 338
 339     /**
 340      * Determine number of segments and number of elements
 341      * sent per operation
 342      */
 343     typelng = datatype->size();
 344
 345     COLL_TUNED_COMPUTED_SEGCOUNT( segsize, typelng, segcount );
 346
 347     return smpi_coll_tuned_ompi_reduce_generic( sendbuf, recvbuf, count, datatype,
 348                                            op, root, comm,
 349                                            ompi_coll_tuned_topo_build_chain(fanout, comm, root),
 350                                            segcount, 0 );
 351 }
 352
 353
 354 int Coll_reduce_ompi_pipeline::reduce(const void *sendbuf, void *recvbuf,
 355                                            int count, MPI_Datatype datatype,
 356                                            MPI_Op  op, int root,
 357                                            MPI_Comm  comm  )
 358 {
 359
 360     uint32_t segsize;
 361     int segcount = count;
 362     size_t typelng;
 363 //    COLL_TUNED_UPDATE_PIPELINE( comm, tuned_module, root );
 364
 365     /**
 366      * Determine number of segments and number of elements
 367      * sent per operation
 368      */
 369     const double a2 =  0.0410 / 1024.0; /* [1/B] */
 370     const double b2 =  9.7128;
 371     const double a4 =  0.0033 / 1024.0; /* [1/B] */
 372     const double b4 =  1.6761;
 373     typelng= datatype->size();
 374     int communicator_size = comm->size();
 375     size_t message_size = typelng * count;
 376
 377     if (communicator_size > (a2 * message_size + b2)) {
 378         // Pipeline_1K
 379         segsize = 1024;
 380     }else if (communicator_size > (a4 * message_size + b4)) {
 381         // Pipeline_32K
 382         segsize = 32*1024;
 383     } else {
 384         // Pipeline_64K
 385         segsize = 64*1024;
 386     }
 387
 388     XBT_DEBUG("coll:tuned:reduce_intra_pipeline rank %d ss %5u", comm->rank(), segsize);
 389
 390     COLL_TUNED_COMPUTED_SEGCOUNT( segsize, typelng, segcount );
 391
 392     return smpi_coll_tuned_ompi_reduce_generic( sendbuf, recvbuf, count, datatype,
 393                                            op, root, comm,
 394                                            ompi_coll_tuned_topo_build_chain( 1, comm, root),
 395                                            segcount, 0);
 396 }
 397
 398 int Coll_reduce_ompi_binary::reduce(const void *sendbuf, void *recvbuf,
 399                                          int count, MPI_Datatype datatype,
 400                                          MPI_Op  op, int root,
 401                                          MPI_Comm  comm)
 402 {
 403     uint32_t segsize;
 404     int segcount = count;
 405     size_t typelng;
 406
 407
 408
 409     /**
 410      * Determine number of segments and number of elements
 411      * sent per operation
 412      */
 413     typelng=datatype->size();
 414
 415         // Binary_32K
 416     segsize = 32*1024;
 417
 418     XBT_DEBUG("coll:tuned:reduce_intra_binary rank %d ss %5u", comm->rank(), segsize);
 419
 420     COLL_TUNED_COMPUTED_SEGCOUNT( segsize, typelng, segcount );
 421
 422     return smpi_coll_tuned_ompi_reduce_generic( sendbuf, recvbuf, count, datatype,
 423                                            op, root, comm,
 424                                            ompi_coll_tuned_topo_build_tree(2, comm, root),
 425                                            segcount, 0);
 426 }
 427
 428 int Coll_reduce_ompi_binomial::reduce(const void *sendbuf, void *recvbuf,
 429                                            int count, MPI_Datatype datatype,
 430                                            MPI_Op  op, int root,
 431                                            MPI_Comm  comm)
 432 {
 433
 434     uint32_t segsize=0;
 435     int segcount = count;
 436     size_t typelng;
 437
 438     const double a1 =  0.6016 / 1024.0; /* [1/B] */
 439     const double b1 =  1.3496;
 440
 441 //    COLL_TUNED_UPDATE_IN_ORDER_BMTREE( comm, tuned_module, root );
 442
 443     /**
 444      * Determine number of segments and number of elements
 445      * sent per operation
 446      */
 447     typelng= datatype->size();
 448     int communicator_size = comm->size();
 449     size_t message_size = typelng * count;
 450     if (((communicator_size < 8) && (message_size < 20480)) ||
 451                (message_size < 2048) || (count <= 1)) {
 452         /* Binomial_0K */
 453         segsize = 0;
 454     } else if (communicator_size > (a1 * message_size + b1)) {
 455         // Binomial_1K
 456         segsize = 1024;
 457     }
 458
 459     XBT_DEBUG("coll:tuned:reduce_intra_binomial rank %d ss %5u", comm->rank(), segsize);
 460     COLL_TUNED_COMPUTED_SEGCOUNT( segsize, typelng, segcount );
 461
 462     return smpi_coll_tuned_ompi_reduce_generic( sendbuf, recvbuf, count, datatype,
 463                                            op, root, comm,
 464                                            ompi_coll_tuned_topo_build_in_order_bmtree(comm, root),
 465                                            segcount, 0);
 466 }
 467
 468 /*
 469  * reduce_intra_in_order_binary
 470  *
 471  * Function:      Logarithmic reduce operation for non-commutative operations.
 472  * Acecpts:       same as MPI_Reduce()
 473  * Returns:       MPI_SUCCESS or error code
 474  */
 475 int Coll_reduce_ompi_in_order_binary::reduce(const void *sendbuf, void *recvbuf,
 476                                                   int count,
 477                                                   MPI_Datatype datatype,
 478                                                   MPI_Op  op, int root,
 479                                                   MPI_Comm  comm)
 480 {
 481     uint32_t segsize=0;
 482     int ret;
 483     int rank, size, io_root;
 484     int segcount = count;
 485     void *use_this_sendbuf = NULL, *use_this_recvbuf = NULL;
 486     size_t typelng;
 487
 488     rank = comm->rank();
 489     size = comm->size();
 490     XBT_DEBUG("coll:tuned:reduce_intra_in_order_binary rank %d ss %5u", rank, segsize);
 491
 492     /**
 493      * Determine number of segments and number of elements
 494      * sent per operation
 495      */
 496     typelng=datatype->size();
 497     COLL_TUNED_COMPUTED_SEGCOUNT( segsize, typelng, segcount );
 498
 499     /* An in-order binary tree must use root (size-1) to preserve the order of
 500        operations.  Thus, if root is not rank (size - 1), then we must handle
 501        1. MPI_IN_PLACE option on real root, and
 502        2. we must allocate temporary recvbuf on rank (size - 1).
 503        Note that generic function must be careful not to switch order of
 504        operations for non-commutative ops.
 505     */
 506     io_root = size - 1;
 507     use_this_sendbuf = const_cast<void*>(sendbuf);
 508     use_this_recvbuf = recvbuf;
 509     if (io_root != root) {
 510         ptrdiff_t text, ext;
 511         char *tmpbuf = NULL;
 512
 513         ext=datatype->get_extent();
 514         text=datatype->get_extent();
 515
 516         if ((root == rank) && (MPI_IN_PLACE == sendbuf)) {
 517             tmpbuf = (char *) smpi_get_tmp_sendbuffer(text + (count - 1) * ext);
 518             if (NULL == tmpbuf) {
 519                 return MPI_ERR_INTERN;
 520             }
 521             Datatype::copy (
 522                                                 (char*)recvbuf, count, datatype,
 523                                                 (char*)tmpbuf, count, datatype);
 524             use_this_sendbuf = tmpbuf;
 525         } else if (io_root == rank) {
 526             tmpbuf = (char *) smpi_get_tmp_recvbuffer(text + (count - 1) * ext);
 527             if (NULL == tmpbuf) {
 528                 return MPI_ERR_INTERN;
 529             }
 530             use_this_recvbuf = tmpbuf;
 531         }
 532     }
 533
 534     /* Use generic reduce with in-order binary tree topology and io_root */
 535     ret = smpi_coll_tuned_ompi_reduce_generic( use_this_sendbuf, use_this_recvbuf, count, datatype,
 536                                           op, io_root, comm,
 537                                           ompi_coll_tuned_topo_build_in_order_bintree(comm),
 538                                           segcount, 0 );
 539     if (MPI_SUCCESS != ret) { return ret; }
 540
 541     /* Clean up */
 542     if (io_root != root) {
 543         if (root == rank) {
 544             /* Receive result from rank io_root to recvbuf */
 545             Request::recv(recvbuf, count, datatype, io_root,
 546                                     COLL_TAG_REDUCE, comm,
 547                                     MPI_STATUS_IGNORE);
 548             if (MPI_IN_PLACE == sendbuf) {
 549               smpi_free_tmp_buffer(use_this_sendbuf);
 550             }
 551
 552         } else if (io_root == rank) {
 553             /* Send result from use_this_recvbuf to root */
 554             Request::send(use_this_recvbuf, count, datatype, root,
 555                                     COLL_TAG_REDUCE,
 556                                     comm);
 557             smpi_free_tmp_buffer(use_this_recvbuf);
 558         }
 559     }
 560
 561     return MPI_SUCCESS;
 562 }
 563
 564 /*
 565  * Linear functions are copied from the BASIC coll module
 566  * they do not segment the message and are simple implementations
 567  * but for some small number of nodes and/or small data sizes they
 568  * are just as fast as tuned/tree based segmenting operations
 569  * and as such may be selected by the decision functions
 570  * These are copied into this module due to the way we select modules
 571  * in V1. i.e. in V2 we will handle this differently and so will not
 572  * have to duplicate code.
 573  * GEF Oct05 after asking Jeff.
 574  */
 575
 576 /* copied function (with appropriate renaming) starts here */
 577
 578 /*
 579  *  reduce_lin_intra
 580  *
 581  *  Function:   - reduction using O(N) algorithm
 582  *  Accepts:    - same as MPI_Reduce()
 583  *  Returns:    - MPI_SUCCESS or error code
 584  */
 585
 586 int
 587 Coll_reduce_ompi_basic_linear::reduce(const void *sbuf, void *rbuf, int count,
 588                                           MPI_Datatype dtype,
 589                                           MPI_Op op,
 590                                           int root,
 591                                           MPI_Comm comm)
 592 {
 593     int i, rank, size;
 594     ptrdiff_t true_extent, lb, extent;
 595     char *free_buffer = NULL;
 596     char *pml_buffer = NULL;
 597     char *inplace_temp = NULL;
 598     char *inbuf;
 599
 600     /* Initialize */
 601
 602     rank = comm->rank();
 603     size = comm->size();
 604
 605     XBT_DEBUG("coll:tuned:reduce_intra_basic_linear rank %d", rank);
 606
 607     /* If not root, send data to the root. */
 608
 609     if (rank != root) {
 610         Request::send(sbuf, count, dtype, root,
 611                                 COLL_TAG_REDUCE,
 612                                 comm);
 613         return MPI_SUCCESS;
 614     }
 615
 616     /* see discussion in ompi_coll_basic_reduce_lin_intra about
 617        extent and true extent */
 618     /* for reducing buffer allocation lengths.... */
 619
 620     dtype->extent(&lb, &extent);
 621     true_extent = dtype->get_extent();
 622
 623     if (MPI_IN_PLACE == sbuf) {
 624         sbuf = rbuf;
 625         inplace_temp = (char*)smpi_get_tmp_recvbuffer(true_extent + (count - 1) * extent);
 626         if (NULL == inplace_temp) {
 627             return -1;
 628         }
 629         rbuf = inplace_temp - lb;
 630     }
 631
 632     if (size > 1) {
 633         free_buffer = (char*)smpi_get_tmp_recvbuffer(true_extent + (count - 1) * extent);
 634         pml_buffer = free_buffer - lb;
 635     }
 636
 637     /* Initialize the receive buffer. */
 638
 639     if (rank == (size - 1)) {
 640         Datatype::copy((char*)sbuf, count, dtype,(char*)rbuf, count, dtype);
 641     } else {
 642         Request::recv(rbuf, count, dtype, size - 1,
 643                                 COLL_TAG_REDUCE, comm,
 644                                 MPI_STATUS_IGNORE);
 645     }
 646
 647     /* Loop receiving and calling reduction function (C or Fortran). */
 648
 649     for (i = size - 2; i >= 0; --i) {
 650         if (rank == i) {
 651             inbuf = (char*)sbuf;
 652         } else {
 653             Request::recv(pml_buffer, count, dtype, i,
 654                                     COLL_TAG_REDUCE, comm,
 655                                     MPI_STATUS_IGNORE);
 656             inbuf = pml_buffer;
 657         }
 658
 659         /* Perform the reduction */
 660         if(op!=MPI_OP_NULL) op->apply( inbuf, rbuf, &count, dtype);
 661     }
 662
 663     if (NULL != inplace_temp) {
 664         Datatype::copy(inplace_temp, count, dtype,(char*)sbuf
 665                                                   ,count , dtype);
 666         smpi_free_tmp_buffer(inplace_temp);
 667     }
 668     if (NULL != free_buffer) {
 669         smpi_free_tmp_buffer(free_buffer);
 670     }
 671
 672     /* All done */
 673     return MPI_SUCCESS;
 674 }
 675
 676 /* copied function (with appropriate renaming) ends here */
 677
 678
 679 }
 680 }